Automatic cooking control system for a microwave oven

ABSTRACT

An automatic cooking control system for a microwave oven having a turntable utilizes an initial operation process which makes the air temperature of a heating chamber by uniformed only operating a fan. The automatic cooking control system further utilizes first stage heating operation process which actuates a magnetron after the initial operation process. The first stage process detects, an outflow air temperature, stores it as a present temperature, and obtains an arithmetical mean of the present temperature and a previous temperature detected prior to a half rotational period of the turntable. The first stage heating operation process is carried out until the arithmetical mean is raised as much as a predetermined value established according to the kind of food being cooked. The system also utilizes a second stage heating operation process which is carried out for a time period that is obtained by multiplying the first stage heating time by a predetermined value established by the kind of food being cooked. This system can automatically cook food with the correct heating time even though the outflow air temperature detected by a temperature sensor oscillates due to the rotation of the turntable.

BACKGROUND OF THE INVENTION

The present invention relates to an automatic cooking control system fora microwave oven with a turntable which can automatically cook food byutilizing a temperature sensor that detects the temperature of airflowing out of a heating chamber, and more particularly, to an automaticcooking control system which is able to automatically cook food byestablishing a heating time of for the food to be cooked even though anoutflow air temperature detected at the temperature sensor oscillatesdue to the rotation of the turntable.

In general, a microwave oven which cooks food automatically isconstructed, as shown in FIG. 1, with a microcomputer 1 which controlsthe whole operation of a microwave oven, a power source 2 which supplieselectric power under the control of the microcomputer 1, a magnetron 3which generates microwave energy upon actuation by electric power fromthe power source 2, a heating chamber 4 which heats the food positionedon a turntable 4A with the microwave energy generated for the magnetron3, a fan 5 which blows air through an air inlet 4B into the heatingchamber 4, a temperature sensor 6 which detects the temperature of airflowing out through an air outlet 4C of the heating chamber 4, and ananalog/digital converter 7 which converts the signal of outflow airtemperature detected by the temperature sensor 6 into a digital signaland inputs the converted signal into the microcomputer 1.

With the conventional microwave oven constructed as described above,when a food to be cooked is put onto a turntable 4A of a heating chamber4 and an automatic cooking is started by pressing a cooking startbutton, a microcomputer 1 begins to execute an initial operation for apredetermined time t₁ as shown in FIGS. 2 and 3. A fan 5 is actuated forabout sixteen seconds to blow air through an air inlet 4B into theheating chamber 4 so that the air temperature of the heating chamber 4can be made uniformed. The temperature of the air flowing out of the airoutlet 4C is detected by a temperature sensor 6. The detectedtemperature signal is then converted into a digital signal by theanalog/digital converter 7.

When a predetermined time t₁ has elapsed, the microcomputer 1 receivesand stores the signal of the present temperature T₁ which is outputtedfrom the analog/digital converter 7. The microcomputer 1 controls theactuation of the power source 2, The food positioned on the turntable 4Aof the heating chamber 4 is heated by microwave energy generated by themagnetron 3. Since the temperature of air flowing out of the heatingchamber 4 through the air outlet 4C is gradually raised according to theheating of food, the temperature detection signal, which is detected bythe temperature sensor 6 and inputted to the microcomputer 1 through theanalog/digital converter 7, is also gradually raised.

The temperature increment is raised as much as a predetermined value ΔT.The temperature detected at the temperature sensor 6 is raised as muchas a predetermined temperature T₂ so that when the temperature incrementbecomes a predetermined value ΔT, microcomputer 1 finishes a first stageheating operation and starts to execute a second stage heating.

In summary, the conventional automatic cooking control is executedutilizing a method having the steps of: storing a time t₂ of a firststage heating; calculating a second stage heating time t₃ by multiplyingthe first stage heating time t₂ by a predetermined value α establishedin accordance with the kind of food to be cooked; heating the food bycontinuously actuating the magnetron 3 for the second stage heating timet₃ ; and completing the cooking of the food by stopping the actuation ofmagnetron 3 and fan 5 when the second stage heating time t₃ has elapsed.

In such an automatic cooking control method, since the geometricalcenter of the turntable 4A and the temperature-responsive center of thefood to be cooked in the course of the rotation of the turntable 4A arenot in precise accord with each other, the temperature characteristic ofoutflow air detected by the temperature sensor 6 oscillates.

FIG. 4 is a graph showing a temperature response characteristic of theoutflow air when cooking egg custard comprising two eggs with two cupsof milk. The temperature response characteristic of outflow airoscillates causing the first stage heating time to become short and thesecond stage heating time to become short, thereby causing the automaticcooking process; not to be correctly performed.

The outflow air temperature oscillates as shown in FIG. 5. The firststage heating process is finished at the time t_(a), but not the desiredtime causing the first stage heating time to be shortened as much as apredetermined time Δt₁. Thus the second stage heating time also becomesshort, thereby making it impossible to execute correctly the automaticcooking of food.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide anautomatic cooking control system which is able to correctly execute theautomatic cooking of food by accurately determining a first stageheating time, even though the outflow air temperature flowing out of theheating chamber oscillates due to the rotation of a turntable. The aboveobject of the present invention is realized by determining whether ornot the first stage heating operation is finished. This determination isaccomplished by obtaining an arithmetical mean of the outflow airtemperatures detected at present and a predetermined time before.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the configuration of aconventional microwave oven;

FIG. 2 is a flow chart of a microcomputer according to a conventionalautomatic cooking control system;

FIG. 3 is a graph illustrating the change of outflow air temperatureaccording to the conventional automatic cooking control system;

FIG. 4 is a graph illustrating the temperature change characteristic ofoutflow air when automatic cooking of a food;

FIG. 5 is a graph illustrating the errors of first stage heating processaccording to the conventional automatic cooking control system;

FIG. 6 is a graph for explaining the automatic cooking control system ofthe present invention; and

FIG. 7 is a flow chart of a microcomputer according to the automaticcooking control system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

To begin with, the method for obtaining an arithmetical mean of theoutflow air temperatures detected at present and a predetermined timebefore using the below, numerical formulas will be explained.

A temperature response which oscillates and has a constant period, canbe represented by the following numerical expression: ##EQU1##

Wherein,

y is a temperature,

k is a gradient,

t is a time,

C is a constant,

A is an amplitude, and

T_(V) is a period.

Accordingly, the arithmetical mean for a temperature of an arbitrarypoint of time t=t₁₁ and a temperature of a point of time ##EQU2## whichis prior to the arbitrary point of time follows. ##EQU3##

As can be seen from the above formula, when the temperatures detected atthe time t₁₁ and ##EQU4## respectively, are summed up to determine thearithmetical mean, the oscillating portion is removed and thetemperature becomes constant.

An error E compared with a normal condition is represented as follows;##EQU5##

A temperature increasing rate of outflow air, a gradient k, is veryslow; however, the rotational period of a turntable 4A, that is, theoscillation period T_(V) of the temperature, is relatively quick.According the error E becomes small and substantially about 20% comparedwith an error according to an oscillation of temperature.

FIG. 7 is a signal flow chart of a the microcomputer 1 according to theautomatic cooking control system of the present invention which executesa first stage heating by obtaining the above described arithmetical meanand a second stage heating. As shown in the drawing, when a user putsfood to be cooked on a turntable 4A of a heating chamber 4, and acooking operation is started by pressing a cooking start button, a themicrocomputer 1 operates an initial operation as usual. Themicrocomputer 1 only actuates a fan 5 to make the air temperature withinthe heating chamber 4 uniformed, when a predetermined time t₁ haselapsed, the outflow air temperature is detected and stored in a memoryMI₁. The outflow air temperature stored in the memory MI₁ is stored inmemories M₁ -M₅, a food is then heated upon actuation of magnetron 3.

After the actuation of the magnetron 3, the microcomputer 1 detects theoutflow air temperature at a constant time interval, for example, 1second interval, and stores it in a memory MI₂. The microcomputer 1 thencomputes the arithmetical mean of the temperatures stored in said memoryMI₂ and a memory M₅ by a numerical formula 1/2(MI₂ +M₅) and stores theproduct in a memory MI₃.

Whether or not the outflow air temperature is raised as much as apredetermined value ΔT is determined by subtracting the value stored ina memory MI₁ from the temperature stored in the memory MI₃. If not, thetemperatures stored in memories M₄ -M₁ are shifted to memories M₅ -M₂,respectively and stored therein. The present outflow air temperaturestored in the memory MI₂ is then stored in the memory M₁. After onesecond has elapsed, the outflow air temperature is detected and storedin the memory MI₂, the arithmetical mean of the temperatures stored inthe memory MI₂ and the memory MI₅ is again calculated. Thereafter, it isdetermined whether the outflow air temperature is raised as much as apredetermined value ΔT. The above process is repeated until the outflowair temperature is raised as much as a predetermined value ΔT.

Thus, when the outflow air temperature is raised as much as the valueΔT, the first stage heating is completed. Then, the second stage heatingtime t₃ is calculated by multiplying the first stage heating time by apredetermined value α which is established in accordance with the kindof food being. The magnetron 3 is continuously actuated for the secondstage heating time t₃ to heat a food. When the second stage heating timet₃ has elapsed, the cooking of food is finished by stopping theactuation of magnetron 3 and fan 5.

As described above, the present invention has the advantage that theautomatic cooking food is very correctly performed by accuratelydetermining the first stage heating time which is executed in a mannerthat by discriminating whether the outflow air temperature is raised asmuch as a predetermined value after obtaining an arithmetical mean ofthe outflow air temperatures detected at present and a predeterminedtime before.

What is claimed is:
 1. A method of automatically cooking in a microwaveoven having a heating chamber, a magnetron and a turntable comprisingthe steps of:(a) measuring and storing a first temperature of airflowing out of the heating chamber; (b) actuating the magnetron; (c)measuring and storing a second temperature of the air flowing out of theheating chamber after a time delay of one second; (d) calculating anarithmetic mean of the first and second temperature; (e) determining ifa difference between the first temperature and the arithmetic meancalculated in said step (d) is equal to a predetermined temperatureincrement, the amount of time between the actuation of the magnetron andthe quality determined in said step (e) defining a first period of time;(f) calculating an additional cooking time by multiplying the firstperiod of time by a predetermined coefficient when said step (e)determines that the difference is equal to the predetermined temperatureincrement; and (g) actuating the magnetron for the additional cookingtime.
 2. The method as claimed in claim 1, further comprising the stepof:(h) actuating a fan of the microwave oven prior to executing saidstep (a).
 3. The method as claimed in claim 1, further comprising thestep of:(h) repeating said steps (c), (d), and (e) when said step (e)determines that the difference is not equal to the predeterminedtemperature increment.
 4. The method as claimed in claim 3, wherein saidstep (a) stores the first temperature in a memory of the microwave oven,the memory having five memory locations, the fifth memory locationrepresenting the first temperature.
 5. The method as claimed in claim 4,wherein said step (h) comprises the steps of:(i) shifting contents ofeach memory location up one location in the memory; and (j) storing thesecond temperature measured in said step (c) in the first memorylocation.
 6. The method as claimed in claim 1, wherein said step (d)calculates the arithmetic mean by multiplying a gradient by a time andadding a constant to the product, a product calculated by multiplyingthe gradient by a quarter period of time is then subtracted from the sumthereby calculating the arithmetic mean.